Efter en lang tur føles mine ben tunge, så jeg sætter mig ned.

Yes. Efter means after, and en lang tur means a long walk / a long trip / a long outing, depending on context.

So:

• efter = after
• en lang tur = a long walk / trip

Starting the sentence with Efter en lang tur sets the scene first, just like English often does with After a long walk, ...

Because tur is a common gender noun in Danish, so it takes en, not et.

The adjective must agree with that form:

• en lang tur
• et langt løb

So:

• lang is used with en-words
• langt is used with et-words

That is why tur gives en lang tur.

Føles means feel(s), as in to feel in the sense of seeming or physically feeling a certain way.

So:

• Mine ben føles tunge = My legs feel heavy
• Mine ben er tunge = My legs are heavy

Both can be possible, but føles focuses more on the speaker’s experience or sensation. It sounds more natural here because the sentence is about how the legs feel after exertion.

Because ben is plural here: legs.

Danish possessives change depending on gender/number:

• min = my, for singular common gender
• mit = my, for singular neuter
• mine = my, for plural

Examples:

• min stol = my chair
• mit hus = my house
• mine ben = my legs

So mine is required because there are two legs, or at least the noun is plural.

Because ben is one of those Danish nouns whose singular and plural can look the same.

• et ben = a leg
• flere ben = several legs

You usually tell from the surrounding words:

• mit ben = my leg
• mine ben = my legs

So in this sentence, mine tells you clearly that it is plural.

Because tunge is the plural form of the adjective tung.

Since ben is plural, the adjective must also be plural:

• et tungt ben = a heavy leg
• tunge ben = heavy legs

Here we have mine ben tunge, so plural agreement is needed.

Because the adjective here comes after the verb føles, not directly before the noun.

Compare these:

• mine tunge ben = my heavy legs
  
  • adjective before the noun
• mine ben føles tunge = my legs feel heavy
  
  • adjective after the verb

After verbs like være or føles, Danish often uses the adjective as a kind of description of the subject. In that position, it is not placed before the noun.

Yes. Here så means so, therefore, or as a result.

It connects the two ideas:

• After a long walk, my legs feel heavy,
• so I sit down.

Be careful, though: så can also mean then in other contexts. The exact meaning depends on the sentence.

This is a reflexive construction.

At sætte sig ned means to sit down or literally to set oneself down.

So:

• jeg sætter mig ned = I sit down
• du sætter dig ned = you sit down
• han sætter sig ned = he sits down

The reflexive pronoun changes with the subject:

• mig = myself
• dig = yourself
• sig = himself/herself/themselves (in reflexive use)

You cannot normally leave it out here. Sætte sig ned is a fixed expression.

Good question. They are not the same.

• jeg sætter mig ned = I sit down
  This describes the action of moving into a sitting position.
• jeg sidder = I am sitting / I sit
  This describes the state of already being seated.

So in this sentence, sætter mig ned makes sense because the heavy legs lead to the action of sitting down.

Because after så in this sentence, Danish uses normal main-clause word order:

• så jeg sætter mig ned

That means:

1. conjunction
2. subject
3. verb

So:

• så = so
• jeg = I
• sætter = sit down / set

If the sentence had started directly with another element in the second clause, then the verb might come before the subject. But here jeg comes right after så, so the order is straightforward.

Because Danish verbs do not change form for singular and plural subjects in the present tense.

So the same present-tense verb form is used with all persons and numbers:

• jeg føler
• du føler
• han føler
• vi føler

And with føles:

• benet føles tungt = the leg feels heavy
• benene føles tunge = the legs feel heavy

The verb stays føles. The adjective is what changes to show singular or plural.

Yes, often you could.

• tur is broader and can mean a trip, outing, walk, or journey depending on context.
• gåtur specifically means walk.

So:

• Efter en lang tur = after a long walk/trip/outing
• Efter en lang gåtur = after a long walk

If the context is definitely walking, gåtur is more specific. Tur is more general and very common in everyday Danish.